1. Field of the Invention
This invention relates to a core to be utilized in a push-pull cable assembly and, more specifically, to such a core which has a self-lubricating coating rigidly bonded to an interior metallic wire member.
2. Description of the Prior Art
There have heretofore been utilized a number of push-pull control cables which have a metallic core member which is coated with a self-lubricating material. For example, U.S. Pat. No. 3,176,538 and British Pat. No. 864,421 disclose such cables where polytetrafluoroethylene is utilized to provide the coating.
However, in these and other cables heretofore employed there has generally been a problem encountered when attempting to bond the polytetrafluoroethylene to the metallic core member. This is understandable when one considers that polytetrafluoroethylene is chosen in the first place for its low coefficient of sliding friction. The general problem with the bonding qualities of this material appears to stem from the fact that polytetrafluoroethylene cannot be melt-formed into finished products by conventional thermoplastic molding techniques but must be extruded or molded by techniques resembling those used for powdered metallurgy. Polytetrafluoroethylene becomes a gel at a transition temperature of about 620.degree. F. to 640.degree. F. and has an extremely high melt viscosity. Accordingly, polytetrafluoroethylene is usually provided in the form of particles which do not flow and melt together, but sinter, and sintered polytetrafluoroethylene does not readily adhere to a metallic surface.
As a result, it has generally been the practice in the past when providing a polytetrafluoroethylene coating to use either an extrusion method or a method of wrapping the metallic core material with an unsintered polytetrafluoroethylene tape which is later sintered. The tape has by necessity been overlapped so that, in both cases, the primary bonding that is produced by the sintering is the bonding of polytetrafluoroethylene to itself as it encircles the metallic core member. It has been observed in articles formed in this manner that any break in the coating endangers the integrity of the coating. The tape, for example, can be unwound because of the lack of bonding to the metal surface and the general tendency of the polytetrafluoroethylene to fail in areas of single thickness of tape which are located between the overlapped regions having two thicknesses of tape. The general lack of bonding of the polytetrafluoroethylene coating to the metal core member has been further evidenced by the existence of air pockets between the coating and the metallic core member. Consequently, even though the polytetrafluoroethylene coatings used on cable cores in the past have provided an excellent low-friction, self-lubricating surface, their reliability for prolonged or heavy-duty operations has been limited.